As technical means for forming silicon carbide (SiC) on the surface of a silicon substrate, there has heretofore been known the Chemical Vapor Deposition (CVD) means using saturated aliphatic hydrocarbons and a homologue of silicon tetrachloride as raw materials. For example, a technique for growing an SiC film on the surface of an Si substrate through the CVD method using propane (C3H8) and trichlorosilane (SiHCl3) as raw materials (refer, for example, to “Silicon-based Heterodevice,” Seijiro Furukawa & Yoshihito Amamiya, Maruzen Co., Ltd., Jul. 30, 1991, pp. 91-93).
As a simpler method for forming silicon carbide, there has heretofore been known a method of carbonizing the surface of a silicon substrate using an unsaturated hydrocarbon gas, such as acetylene (C2H2) (refer, for example, to the prior art just mentioned above). There has also been known means for forming a silicon carbide film by irradiating the surface of a substrate with an acetylene gas within a Molecular Beam Epitaxial (MBE) apparatus retained in high vacuum of 10−5 Pa to carbonize the surface of the substrate (refer, for example, to “Journal of Crystal Growth,” T. Ohachi et al., the Netherlands, Vol. 275 (1-2), 2005, pp. e1215-e1221).
In the prior art means for simply carbonizing the silicon surface, however, the carbonization does not always be promoted satisfactorily homogeneously on the surface of a silicon substrate. That is to say, a silicon carbide layer having a uniform thickness cannot reliably be formed. This is problematic. A partial region of the silicon substrate surface is not coated with a silicon carbide layer, but exposed to an atmosphere. Even when such a layer inhomogeneous in configuration is used as an under layer, therefore, formation of an upper layer having a uniformly unified crystal shape will fail to produce.
A cubic silicon carbide crystal (3C—SiC; lattice constant=0.436 nm) has substantially the same lattice constant as a cubic gallium nitride crystal (GaN; lattice constant=0.451 nm). In addition, the lattice spacing of the (110) plane of a cubic silicon carbide (=0.308 nm) substantially conforms to that of the a-axis of a hexagonal GaN crystal (0.318 nm). Therefore, a cubic silicon carbide crystal layer can constitute a lattice-matched under layer for growing thereon a cubic or hexagonal gallium nitride crystal upper layer. According to the prior art techniques, however, it is impossible as described above to reliably form a silicon carbide layer uniformly coating the entire surface of the silicon substrate. For this reason, when it is intended to form a GaN layer using a silicon carbide layer excellent in lattice matching as an under layer, the silicon carbide layer cannot constitute an under layer capable of forming thereon a Group III nitride semiconductor layer having homogeneous crystalline characteristics. This is also problematic.
The present invention has been proposed in view of the above. The object of the present invention is to provide a method for producing a silicon carbide layer capable of uniformly coating the surface of a silicon substrate through irradiation of the silicon substrate surface of a gas comprising a saturated aliphatic hydrocarbon or an unsaturated hydrocarbon, provide a gallium nitride-based semiconductor device formed on the silicon carbide layer and provide a silicon substrate including the silicon carbide layer.